Too short is obvious that the condom isn't long enough to roll down to the base. If there's like half an inch uncovered, then it doesn't matter too much.
For tightness, think about clothes, like socks or jeans. If they're too tight, they'll dig into the skin, if they're too loose, they will be slipping down.
Same way with a condom, if it's too tight, you will notice the edge of it digging into you and it might be that you need to slightly squeeze yourself or stretch the condom a bit as you're putting it on, it doesn't just roll on nicely. You may also have it break while using it if it's too tight. If you don't lik condoms because you "can't feel anything" with them on. It's a good indication they're too tight.
If it's loose, you may notice that it's not all against the skin, having some loose condom bunching up. You can also push on the edge of it from the bottom. Imagine a loose ring. It may not look like it's too big for your finger from above because gravity. But if you push it up then you see how much free space there is between the ring and the finger.
If it helps, this page explains it with some images too
https://www.theyfit.co.uk/pages/comparison-of-condom-sizes